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Kinetic Study on Nucleophilic Substitution Reactions of Aryl Diphenylphosphinates with Butane‐2,3‐dione Monoximate and Aryloxide Anions: Reaction Mechanism and Origin of the α‐Effect
Author(s) -
Um IkHwan,
Han JeongYoon
Publication year - 2016
Publication title -
bulletin of the korean chemical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.237
H-Index - 59
ISSN - 1229-5949
DOI - 10.1002/bkcs.10836
Subject(s) - chemistry , butane , nucleophilic substitution , aryl , medicinal chemistry , nucleophile , substitution reaction , concerted reaction , reaction mechanism , kinetic energy , stereochemistry , catalysis , alkyl , organic chemistry , physics , quantum mechanics
A kinetic study is reported for nucleophilic substitution reactions of X‐substituted‐phenyl diphenylphosphinates ( 3a–3f ) with butane‐2,3‐dione monoximate (Ox − ) and a series of Y‐substituted‐phenoxide (Y‐ PhO − ) ions in 50 mol % H 2 O /50 mol % DMSO at 25.0 ± 0.1°C. The reactions of 3a–3f with Ox − and 4‐chlorophenoxide (4‐ ClPhO − ) result in linear Brønsted‐type plots with β lg = −0.70 and −0.64, respectively, a typical β lg value for reactions reported previously to proceed through a concerted mechanism. The Brønsted‐type plots for the reactions of 4‐chloro‐2‐nitrophenyl diphenylphosphinate ( 3a ), 4‐nitrophenyl diphenylphosphinate ( 3b ), and 4‐acetylphenyl diphenylphosphinate ( 3d ) with Y‐ PhO − are also linear with β nuc = 0.15–0.35. The current reactions have been concluded to proceed through a concerted mechanism in which the bond formation is much less advanced than the bond rupture in the TS on the basis of the β lg and β nuc values. The α‐effect observed in this study is very small ( i . e ., the k Ox − / k p ‐ ClPhO −ratio = 16.4 – 43.5) and is independent of the leaving‐group basicity. It has been concluded that the α‐effect shown by Ox − in the current reactions is mainly due to desolvation of Ox − in the reaction medium (ground‐state contribution) rather than stabilization of the transition‐state (TS contribution) on the basis of the kinetic results.